Appendix B
Measuring Temperature with Resistive Transducers
©
National Instruments Corporation
B-9
SCXI-1581 User Manual
Resistance/Temperature Characteristic of Thermistors
The resistance-temperature behavior of thermistors is highly dependent
upon the manufacturing process. Therefore, thermistor curves are not
standardized to the extent that thermocouple or RTD curves are
standardized. Typically, thermistor manufacturers supply the
resistance-versus-temperature curves or tables for their particular devices.
You can, however, approximate the thermistor curve relatively accurately
with the Steinhart-Hart equation:
where
T
(°K) is the temperature in degrees Kelvin, equal to
T
(°C) + 273.15.
R
T
is the resistance of the thermistor.
a, b, and c are coefficients obtained from the thermistor manufacturer
or calculated from the resistance-versus-temperature curve.
Creating a Thermistor Virtual Channel Using
NI-DAQmx
To creat an RTD virtual channel using NI-DAQmx, complete the following
steps:
1.
Launch MAX.
2.
Right-click
Data Neighborhood
and select
Create New
.
3.
Select
NI-DAQmx Channel
and click
Next
.
4.
Select
Analog Input»Temperature»
and either
Iex Thermistor
or
Vex Thermistor
and click
Next
.
5.
Select the E Series DAQ device channel to use and click
Next
.
6.
Enter a name for the virtual channel and click
Finish
.
7.
In the configuration screen that appears, select the input range,
I
EX
or
V
EX
source,
I
EX
or
V
EX
value, wire configuration, and enter the values
of
A
,
B
, and
C
.
A
,
B
, and
C
are coefficients obtained from the
thermistor manufacturer or calculated from the
resistance-versus-temperature curve. For
V
EX
thermistors you must
also enter the value of R1, the value of the dropping resistor in ohms.
8.
To test the NI-DAQmx thermistor virtual channel, click the
Test
button.
T(
°
K
)
1
a b ln
R
T
( )
[
]
c ln
R
T
( )
[
]
3
+
+
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